I am currently in the process of building a keyboard (an ergodox). It would be totally awesome to build in an NFC Reader that would do just that.

At the moment I have a Teensy 2.0 to run the thing. I will look into adding the nfc reader, I can't remember how many pins I am currently using on the teensy, so I may have to upgrade the board to a 3.0 or an arduino. I like the idea of making an NFC HID, this way the the password could be used in other applications as well.

Maybe even have a set of passwords on the ring, and hold down a certain ke y at the same time to select the password to input.

I guess if you are putting a group of passwords on it then you would want to encrypt them, and maybe get the keyboard to decrypt them.

What you could do there is put it in the keyboard so that about 1 centimetre worth of edge is accessible, with the rest under whatever else you've got in there. You'd have to test with your ring to figure exactly how/where you're consistently triggering, with the v3 unit I get good reads from the side within 4mm, following the direction of the antenna loop at up to 10mm and over the center of the board for slightly more distance in the right orientation.
You could also play around with disabling the integral antenna and using your own loop antenna with it.